The present invention relates to a touch panel device for detecting the contact of an object, such as a finger or a pen, with the touch panel device, and more particularly relates to a touch panel device for detecting the contact position of the object by detecting attenuation and cutoff of surface acoustic waves (SAWs) by using an excitation element and a receiving element, each constructed by forming electrodes on a piezoelectric body.
With the spread of computer systems, mainly personal computers, there is used a device for inputting new information or giving various instructions to a computer system by pointing at a position on the display screen of a display device on which information is displayed by the computer system, with an object such as a finger or a pen. In order to perform an input operation with respect to the information displayed on the display screen of the display device of a personal computer or the like by a touching method, it is necessary to detect the contact position (pointed position) on the display screen with high accuracy.
Well known examples of a touch panel device for detecting the contact position of an object such as a finger or a pen are a device using a resistance film, and a device using ultrasonic waves. The former device using a resistance film detects a change in the resistance value of the resistance film caused by contact of the object with the resistance film. This device has the advantage of low consumption of power, but has problems in the aspects of the response time, detection performance and durability.
By contrast, in the device using ultrasonic waves, the contact position of an object such as a finger or a pen is detected by propagating surface acoustic waves on a non-piezoelectric substrate, for example, and detecting attenuation of surface acoustic waves caused by contact of the object with the non-piezoelectric substrate. A variety of such touch panel devices have been proposed (for example, Japanese Patent Application Laid-Open Nos. 6-75688/1994 and 10-55240/1998, and Published Japanese Translation of PCT International Application No. 10-504414/1998).
Regarding a touch panel device using surface acoustic waves, there was developed a touch panel device which uses, as transducers, comb-like electrodes (IDTs: inter digital transducers) that can be produced collectively using a photolithography technique. In this touch panel device, an element constructed by forming a comb-like electrode on a piezoelectric body in the form of a thin film is used as an excitation element for exciting a surface acoustic wave and a receiving element for receiving a propagated surface acoustic wave.
FIG. 1 is an illustration showing the configuration of such a conventional touch panel device using comb-like electrodes. In FIG. 1, the numeral 61 represents a rectangular non-piezoelectric substrate. A plurality of excitation elements 62, each constructed by forming a comb-like electrode on a piezoelectric thin film, for exciting surface acoustic waves are arranged in a line on one end of each of the X-direction and the Y-direction of the non-piezoelectric substrate 61. Moreover, a plurality of receiving elements 63, each constructed by forming a comb-like electrode on a piezoelectric thin film, for receiving surface acoustic waves are arranged in a line on the other end of each of the X-direction and the Y-direction of the non-piezoelectric substrate 61.
In this touch panel device, a periodic signal is inputted to each excitation element 62 to excite surface acoustic waves and propagate them on the non-piezoelectric substrate 61, and then the propagated surface acoustic waves are received by the respective receiving elements 63. When an object such as a finger or a pen is in contact with the propagation path of a surface acoustic wave on the non-piezoelectric substrate 61, the surface acoustic wave attenuates. Accordingly, by detecting whether or not there is attenuation in the level of the received signals by the receiving elements 63, it is possible to detect the presence or absence of contact of an object and the contact position thereof.
Besides, in order to continuously detect the presence or absence of contact of an object and the contact position thereof and to improve the resolution of the detection position by increasing the time difference, the present inventor et al. proposed a touch panel device in which the excitation element and the receiving element are arranged so as to propagate surface acoustic waves in an oblique direction (diagonal direction) of the substrate. FIG. 2 is an illustration showing the configuration of such a touch panel device. In FIG. 2, the numeral 71 represents a rectangular non-piezoelectric substrate, and a center portion enclosed by the alternate long and short dash line is a detection region 71a capable of detecting the contact position.
In a frame region 71b outside the detection region 71a, which is a peripheral section of the non-piezoelectric substrate 71, excitation elements 72 are disposed on the upper side and lower side of the substrate 71, while receiving elements 73 are arranged on the left side and right side thereof. Each of the excitation elements 72 and the receiving elements 73 is constructed by forming a comb-like electrode 75 on a piezoelectric body 74 in the form of a thin film (shown by the alternate long and short dash line). The comb-like electrode 75 comprises facing bus electrodes 77, and a plurality of electrode fingers 78 which are extended from the bus electrodes 77 by turns and bent in the middle. In this configuration, lines of a plurality of electrode fingers 78 tilted in two directions from the facing direction of the bus electrodes 77 are formed, thereby realizing simultaneous excitation of surface acoustic waves in two directions and simultaneous reception of surface acoustic waves from two directions. Furthermore, in the frame region 71b, there are provided drawn-round wires 79 for connecting the respective bus electrodes 77 to external circuits.
In such a configuration, surface acoustic waves are excited in two directions by the excitation elements 72, and the excited surface acoustic waves are propagated in two diagonal directions of the non-piezoelectric substrate 71 and then received by the receiving elements 73. Based on the received results, the presence or absence of contact of an object and the contact position thereof are detected.
In addition, the present inventor et al. proposed a touch panel device in which each of the excitation elements and receiving elements is composed of a film-shape piezoelectric body, a comb-like electrode formed on one surface of the piezoelectric body, and a plate electrode (solid electrode) formed on the other surface of the piezoelectric body. FIG. 3 is an illustration showing the configuration of a touch panel device having such a SPT (Single Phased Transducer) electrode structure. In FIG. 3, the numeral 1 represents a rectangular non-piezoelectric substrate, and a center portion enclosed by the alternate long and short dash line is a detection region 1a capable of detecting the contact position.
In a frame region 1b outside the detection region 1a, which is a peripheral section of the non-piezoelectric substrate 1, excitation elements 2 for simultaneously exciting surface acoustic waves in two directions and receiving elements 3 for simultaneously receiving surface acoustic waves from two directions are arranged. Each of these excitation elements 2 and receiving elements 3 is constructed by forming a comb-like electrode 5 on one surface (front surface) of a piezoelectric body 4 in the form of a thin film (indicated by the alternate long and short dash line) and forming a plate electrode (solid electrode) 6 (indicated by the broken line) on the other surface (rear surface) thereof. The comb-like electrode 5 comprises one line of bus electrode 7, and a plurality of electrode fingers 8 which are extended from the bus electrode 7 and bent in V-shape in the middle. Additionally, drawn-round wires 9 from the respective plate electrodes 6 are placed in the frame region 1b. 
In such a configuration, by applying a periodic signal between the comb-like electrode 5 and the plate electrode 6, surface acoustic waves are simultaneously excited in two directions by the excitation elements 2, and the excited surface acoustic waves are propagated in two diagonal directions of the non-piezoelectric substrate 1 and received by the receiving elements 3 (channels 1 through 4 in FIG. 3). Here, when an object such as a finger or a pen is in contact with the propagation path of a surface acoustic wave on the non-piezoelectric substrate 1, the surface acoustic wave attenuates. Therefore, by detecting the presence or absence of attenuation in the level of the received signals by the two receiving elements 3, it is possible to detect the presence or absence of contact of the object and the contact position thereof.
In a touch panel device using surface acoustic waves, it is desired that the excitation strength of surface acoustic wave should not vary greatly depending on the locations. The excitation elements and receiving elements of the touch panel devices using the comb-like electrodes shown in FIG. 2 and FIG. 3 have a structure in which an extremely large number of capacitors are continuously connected.
When an AC voltage signal is applied to the excitation element having such a structure, a delay is caused in the propagation of surface acoustic wave from the proximal end to the distal end of the signal input. Moreover, reflection of the signal occurs at the terminal end, and the reflected wave returns to the proximal end with delay. As a result, there is a difference in the voltage applied to the piezoelectric body between the proximal end and the distal end of the excitation element, thereby causing variations in the applied voltage as shown in FIG. 4. In FIG. 4, the voltage distribution is shown by indicating the distance from the signal input terminal on the abscissa and indicating the applied voltage on the ordinate. Note that the receiving element having the same structure as the excitation element also shows such an uneven voltage distribution.
Accordingly, the strength of the excited surface acoustic wave varies depending on the locations. Moreover, since the surface acoustic wave is propagated in a diagonal direction of the substrate, the propagation distance is not uniform, and therefore the amount of attenuation due to propagation changes according to the propagation distance. In the case where a change in the strength of surface acoustic wave due to variations in voltage and a change in the amount of attenuation according to the propagation distance are taken into account, the received signals in channels 1 through 4 in FIG. 3 have the outputs as shown in FIG. 5, and the change in the output of received signal differs in each channel. For example, the received signals in the region shown by hatching in FIG. 3 will be extremely low outputs. Thus, since the patterns of output strength of received signals in the respective channels are not the same, there is the problem that the contact position of the object cannot be detected accurately and there is a margin for improvement.